hi! the callback is comming just in:<div><br></div><div><div>"Clean up session id and external primitive index"</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>self storePointerUnchecked: 2 ofObject: lit withValue: ConstZero. <- here</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>self storePointerUnchecked: 3 ofObject: lit withValue: ConstZero.</div><div><br></div><div>I know, because I'm debugging with gdb, that writing to that place causes a page fault (target object's page is marked as read only), and the page fault handling mecanism issues the callback to handle it). After all that, the original primExternalCall continues execution, and uses the wrong values of messageSelector, and lkupClass (even if it found the primitive it would write in the wrong place of the cache I think).</div>
<div><br></div><div>I know that the vm has a lot of state and of course you don't want to save everything, but the callback could come in any place, not just primExternalCall, so any variable could be used. I was actually surprised that just saving the active context and creating a new one was enough to save all the state of the VM. Thinking what is enough will not be easy. I tried manually saving and then restoring messageSelector and lkupClass before and after the callback, which solved the problem for some iterations of interpreting, but seemed to corrupt the image, which crashed after some moments. Is there anything else you'd recommend to save to workaround this for now?</div>
<div><br></div><br><div class="gmail_quote">On Thu, Mar 31, 2011 at 12:53 PM, Eliot Miranda <span dir="ltr"><<a href="mailto:eliot.miranda@gmail.com">eliot.miranda@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<br><br><div class="gmail_quote"><div class="im">On Thu, Mar 31, 2011 at 6:11 AM, Javier Pimás <span dir="ltr"><<a href="mailto:elpochodelagente@gmail.com" target="_blank">elpochodelagente@gmail.com</a>></span> wrote:<br>
</div><div><div></div><div class="h5"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi, we are having a problem with callbacks in alien and we would like to see if we are doing something wrong or if it is a bug in the implementation (for the standard old vm).<div><br></div><div>We are receiving the callback just in the middle of a primitiveExternalCall (actually to a function that will fail because the plugin is not present, but i don't think that's important). We pinned it to occur always in the same line, which is </div>
<div><br></div><div><div><span style="white-space:pre-wrap"> </span>longAtput((lit + (BASE_HEADER_SIZE)) + (2 << (SHIFT_FOR_WORD)), ConstZero);</div></div><div><br></div><div>of primitiveExternalCall. When the callback occurs, the thunkEntry is called, which if we understand correctly, saves the active context and runs the interpreter by calling sendInvokeCallbackStackRegistersJmpbuf. The problem is that things like messageSelector and lkupClass, which are global variables are not saved while saving the context, and when the callback returns, the last line of primitiveExternalCall,</div>
<div><br></div><div><div><span style="white-space:pre-wrap"> </span>rewriteMethodCacheSelclassprimIndex(messageSelector, lkupClass, 0);</div>
</div><div><br></div><div>puts a 0 in the wrong place. Also, probably as las message sent was primReturnFromContext:through: (because we just returned from the context), we get a primitiveFailed, but not for the original called function but for primReturnFromContext:through:.</div>
<div><br></div><div>What do you think? are we missing something?</div></blockquote><div><br></div></div></div><div>Hmmm, looking at it I think you must be taking a callback before the external call occurs. Here's how the code reads in Cog:</div>
<div><br></div><div><br></div><div><span style="white-space:pre-wrap"> </span>...</div><div><br></div><div><span style="white-space:pre-wrap"> </span>addr := self ioLoadExternalFunction: functionName + BaseHeaderSize</div>
<div><span style="white-space:pre-wrap"> </span>OfLength: functionLength</div><div><span style="white-space:pre-wrap"> </span>FromModule: moduleName + BaseHeaderSize</div><div><span style="white-space:pre-wrap"> </span>OfLength: moduleLength.</div>
<div><span style="white-space:pre-wrap"> </span>addr = 0</div><div><span style="white-space:pre-wrap"> </span>ifTrue: [index := -1]</div><div><span style="white-space:pre-wrap"> </span>ifFalse: ["add the function to the external primitive table"</div>
<div><span style="white-space:pre-wrap"> </span>index := self addToExternalPrimitiveTable: addr].</div><div><br></div><div><span style="white-space:pre-wrap"> </span>"Store the index (or -1 if failure) back in the literal"</div>
<div><span style="white-space:pre-wrap"> </span>objectMemory storePointerUnchecked: 3 ofObject: lit withValue: (objectMemory integerObjectOf: index).</div><div><br></div><div><span style="white-space:pre-wrap"> </span>"If the function has been successfully loaded cache and call it"</div>
<div><span style="white-space:pre-wrap"> </span>index >= 0</div><div><span style="white-space:pre-wrap"> </span>ifTrue:</div><div><span style="white-space:pre-wrap"> </span>[self rewriteMethodCacheEntryForExternalPrimitiveToFunction: (self cCode: [addr] inSmalltalk: [1000 + index]).</div>
<div><span style="white-space:pre-wrap"> </span>self callExternalPrimitive: addr]</div><div><span style="white-space:pre-wrap"> </span>ifFalse: ["Otherwise void the primitive function and fail"</div>
<div><span style="white-space:pre-wrap"> </span>self rewriteMethodCacheEntryForExternalPrimitiveToFunction: 0.</div><div><span style="white-space:pre-wrap"> </span>^self primitiveFailFor: PrimErrNotFound]</div>
<div><br></div><div>So the rewrite to zero (self rewriteMethodCacheEntryForExternalPrimitiveToFunction: 0) isn't done if no callout is made. Where is your callback comming from? Looks like its comming from the internals of things like ioLoadExternalFunction...</div>
<div><br></div><div>It is hard to save and restore all the VM state around a callback. There's too much of it in the current VM design. Take a look at rewriteMethodCacheEntryForExternalPrimitiveToFunction:. It is written to be fast, using lastMethodCacheProbeWrite to avoid work in rewriting the cache entry if the module and/or function load fails. That's state one doesn't want to have to save and restore around callbacks along with lkupClass, messageSelector. primitiveFunctionPointer, newMethod, framePointer, instructionPointer and stackPointer are already a lot. Tthis needs more thought.</div>
<div class="im">
<div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><br></div><div>Regards,</div><div> Javier.</div><div><br></div><font color="#888888"><div>
<br></div><div><div>-- <br>Javier Pimás<br>Ciudad de Buenos Aires<br>
</div></div>
</font></blockquote></div></div><br>
</blockquote></div><br><br clear="all"><br>-- <br>Javier Pimás<br>Ciudad de Buenos Aires<br>
</div>